1. Field of the Invention
This invention relates generally to thermal ink jet printing, and more particularly, to a thermal ink jet printhead and control arrangement therefor.
2. Description of the Prior Art
As known in the art, thermal ink jet printing systems include printheads which utilize thermal energy selectively produced by heating elements located in capillary-filled ink channels near channel terminating nozzles or apertures to vaporize the ink momentarily and form temporary bubbles on demand. The rapid formation of a temporary bubble causes an ink droplet to be expelled from the printhead and propelled towards a recording medium. The printhead may be incorporated in either a carriage-type printer or a pagewidth-type printer. The carriage-type printer generally has a relatively small printhead containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is reciprocated to print one swath of information at a time on a stationary recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. In contrast, the pagewidth printer includes a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the pagewidth printhead in a direction normal to the printhead length and at a constant speed during the printing process.
The printheads described above may be designed to include from several hundred to several thousand individual ink droplet emitting channels, each channel having a heating element positioned therein. Each of the heating elements includes a pair of end portions or "terminals". A pair of input and output leads or electrodes are normally connected to and extend from these end portions. These electrodes provide a means for selectively introducing electrical signals to the heating elements to initiate the ink vaporization and bubble formation processes. However, due to the geometric constraints of the printhead itself, it is extremely impractical to gain access to both electrodes extending from the heating elements positioned within the array of channels formed in the printhead structure. As a result, one electrode of each heating element is generally connected to a common bus which extends between the array of heating elements and the ink emitting edge of the printhead structure. To achieve high printhead performance, it is desired to minimize the width of the common bus since there are known performance advantages in placing the heating elements as close to the ink emitting edge of the structure as possible. For most arrays, this leads to a difficult tradeoff between bus width and image-dependent voltage drops in the bus.
U.S. Pat. No. 4,458,256 provides an example of an ink jet recording apparatus or structure which utilizes a common bus extending between an array of heating elements positioned within the structure and the ink emitting edge of the structure to provide a common return for electrical signals passed through each of the heating elements thereof. This patent discloses an ink jet recording apparatus which includes a first substrate having a plurality of heating elements positioned thereon and a second substrate having an equal plurality of channels formed therein. The first and second substrates are positioned in abutting contact with one heating element located within one channel. An individual input electrode is connected with one end of each of the heating elements. The other end of each of the heating elements is connected with a common bus which may either wrap around the underside of the first substrate or extend to the side thereof. With this arrangement, an electrical signal provided to the input electrode of a selected heating element is passed through the selected heating element and thereafter through the common bus. It is apparent that, with this arrangement, the common bus must be of a mechanical size sufficient to permit the electrical signal to pass therethrough over the entire length of the bus without the introduction of image-dependent voltage drops therein.
Another example of a thermal ink jet printhead which utilizes a common electrical signal return bus is set forth in U.S. Pat. No. 4,463,359. This patent discloses a printhead having one or more ink filled channels which are replenished by capillary action. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth of the bubbles which causes a quantity of ink to bulge from the nozzle and break off into a droplet at the beginning of the bubble collapse. The current pulses provided to the resistors are thereafter passed through a common bus located along the front and side of the resistor array.
As described, these prior art thermal ink jet printheads each utilize a common bus as the sole return path for electrical signals or pulses passed through selected heating elements of the printhead structure. As a result, the bus itself must be of sufficient mechanical size to carry the full value of the electrical signal or pulse over its entire length in order to prevent the introduction of image-dependent voltage drops therein. In addition, the size requirements of these prior art printhead common bus arrangements preclude optimum placement of the heating elements positioned therein adjacent to the nozzles or apertures of the ink emitting channels.
Therefore, there is generally a need for an improved thermal ink jet printhead structure which overcomes the shortcomings of the prior art. Specifically, the improved thermal ink jet printhead structure must be arranged to provide a common bus connected with the plurality of heating elements of the structure which does not form the sole return path for electrical signals or pulses passed through selected heating elements. This arrangement permits optimum sizing of the common bus and further allows the individual heating elements to be positioned in close proximity to the apertures of the ink emitting channels. In addition, there is a need for a novel control arrangement operable in conjunction with the improved printhead to selectively activate preselected ink emitting channels when it is required to emit an ink droplet and propel the droplet onto a recording medium.